Protection of metals and metal alloys against atmospheric corrosion constitutes a challenge of significant economic importance. There are two distinct corrosion control technologies commonly applied to protect against atmospheric corrosion: conversion coatings and organic coatings. The present invention relates to novel pigment grade host—guest compositions that are suitable for applications in organic coatings and organic primers intended for metal protection. The guest species of the compositions possess corrosion inhibition ability, and inhibition is achieved when the guest species are spontaneously released from the host matrix into aqueous environments in contact with corroding metal substrates.
Pigment grade corrosion inhibitors are generally employed as functional constituents of organic coatings or primers, with organic primers considered the most versatile control technology of metal corrosion under atmospheric conditions.
Currently, when the barrier function of the primer is “lost”, corrosion inhibiting pigments provide the only known protective mechanism at damage sites of metal supported organic coatings. This protective mechanism implies leaching of inhibitor species from coatings while in contact with an aqueous phase and transportation of the inhibitor species by diffusion to specific damage sites of coatings where corrosion occurs. It will be apparent that pigmented primers under atmospheric corrosion conditions function like reservoirs of corrosion inhibitor species, with the reservoirs opening during aqueous events. Likewise, the effectiveness in corrosion inhibition of the primer reservoir depends on the chemical identity, solubility and load (referred to as Pigment Volume Concentration or PVC) of the inhibitor species.
As is well known, pigment grade corrosion inhibitors used in organic primers must contain anionic species with effective inhibitor activity and must be characterized by limited, but effective, solubility in water. For these reasons, Cro4−− is the corrosion inhibitor species preferred in both conversion coating and high performance organic primer technologies applied on metals for protection against atmospheric corrosion.
It is also well known that corrosion inhibitor pigments are comprised of selected inorganic salts, specifically inorganic salts of weak oxi-acids, or electrolytes, with limited solubility in water. It has been noted, that while the anionic constituents comprise the active corrosion inhibitor species of pigments, cationic constituents determine essential properties of the latter, such as solubility, hydrolysis pH, and specific gravity. Such is evident with the chromate series of inhibitor pigments (where CrO4−− is the active inhibitor), which includes Ca, Ba, Sr and Zn-chromates. Current research and development activities in this field are focused on development of effective pigment grade corrosion inhibitors and, specifically, on the development of effective, non-toxic replacements for chromates in high performance organic coatings, which are used as coil and aircraft primers.
Aircraft primers and coil primers are the typical high performance organic coatings that are applied for protection of metals against atmospheric corrosion, most notably for aluminum protection, and especially in the aircraft manufacturing industry. SrCrO4 is the corrosion inhibitor pigment of choice for aircraft and coil primers, and is the standard in the industry. Due to environmental concerns, finding a replacement for chromates in organic coatings constitutes a main objective of contemporary research in this field. Likewise, efforts have been made to expand the application of organic corrosion inhibitors for use in pigment grade compositions for all metals and not just specifically aluminum.
It is generally known, that the number of inorganic anionic corrosion inhibitor species suitable for pigment synthesis and available for chromate replacement is limited essentially to a few, and specifically to MoO4−−, PO4−−−, BO2−, SiO4−− and NCN−−. As a consequence, all commercial non-chromate corrosion inhibitor pigments are molybdates, phosphates, borates, silicates or cyanamides, or combinations of these compounds. It should be noted that, NO2−, a very effective inhibitor, is not available in pigment grades, since all nitrites are too soluble for coating applications. Likewise, it should be noted that some anionic species, such as Cl−, SO4−−, SO3−− and most notably, NO3− to some extent, are known promoters of metal corrosion, rather than inhibitors.
In comparison to CrO4−−, inherent limitations of their corrosion preventing mechanism render these above-specified species less effective inhibitors of corrosion, in general. Consequently, it appears that inorganic chemistry is unable to produce an effective, non-toxic alternative of CrO4−−. In contrast, a large arsenal of organic corrosion inhibitor is known and applied in various corrosion control technologies. Mechanistic shortcomings, excessive solubility in water and/or volatility of most of the known organic inhibitors appear to be the physical properties inconsistent with applications in organic coatings. Thus, requirements for pigment grade inhibitors, delimited by the above suggested reservoir model of pigmented organic coatings, include, among other requirements, a solid consistency, non-volatility, a limited, though effective, solubility in water, high load of the inhibitor species, an efficient inhibitor mechanism, and further having a compatible environmental profile.
An area of interest in this regard has been the development of hydrotalcite based products for corrosion inhibition. As discussed later in more details, hydrotalcite belongs to the family of mixed hydroxides of layered structure, which possess anion-exchange capability and are known to form a considerable number of derivatives containing diverse guest anions.
Hydrotalcite has found various applications, such as Cl− scavenger in plastics and as an acid neutralizer in various systems.
Specifically referring to applications in corrosion inhibition, for instance, Buchheit et al., U.S. Re. No. 35,576, (U.S. Pat. No. 5,266,356) describes the in situ and spontaneous formation of a hydrotalcite—like coatings on aluminum alloys for their corrosion protection. However, the patent is limited in that it does not mention use of inorganic or organic inhibitor anionic species in this context. Kuroda et al., U.S. Pat. No. 5,595,747, demonstrates the effectiveness of hydrotalcite, through its ion exchange property, to hold functional organic anions intended for various applications by slow release, such as releasing an active pesticide. However, Kuroda only references biocidally active compounds and does not suggest use with, or of, corrosion inhibitor compounds.
Miyata et al., U.S. Pat. No. 4,761,188, claims a hydrotalcite based filiform corrosion inhibiting composition containing a number of inorganic and organic anionic species, such as I−, HCO32−, CO32−, CrO42−, ferrocyanide anion, and, respectively, salicilate and oxalate anion. It will be apparent however, that except for CrO42−, and I−, which is known to rather promote corrosion, the rest of the inorganic and organic anionic species claimed within the patent are not credited with and generally are not recognized for corrosion inhibitor activity, specifically in coating applications. Consequently, these hydrotalcite derivatives cannot be considered pigment grade corrosion inhibitors.
As for organic corrosion inhibitors employed in organic coatings, it should be noted that Sinko (U.S. Pat. No. 6,139,610) discloses the application of selected organic compounds in the form of organic-inorganic hybrid corrosion inhibitor pigments intended for chromate replacement in organic coatings. However, Sinko does not mention the applicability of hydrotalcite specifically, for synthesizing pigment grade organic/inorganic ‘hybrid’ inhibitors. It can be concluded that, as of to date, the application of organic corrosion inhibitors in organic coatings has not reached commercial significance.